The present invention relates to methods of and apparatus for regulating cathode current density in electro-plating baths, in particular those of electro-plating installations having at least one controllable power-supply unit by means of which the plating current I or the plating voltage U can be adjusted, in accordance with the optimum current density J.sub.O for the particular electro-plating bath involved and the surface area A of the parts to be electroplated, to the value U.sub.A or I.sub.A which is necessary to produce J.sub.O.
Electrolytes and electroplating baths for use in depositing metal on metallic articles are generally so constituted that they exhibit the optimum deposition characteristics only when the density of the cathode current lies within a specific range. If metal is deposited outside this specific range then the deposits which are made often have characteristics which differ considerably from those of deposits made when the current density is within the proper range. It is therefore desirable for the deposition of metal onto an article to be plated to take place at a specific current density. A specific current density is also a pre-requisite for achieving predetermined coating thicknesses since it is current density alone which determines the charge transfer which takes place per unit of time and per unit of area.
In principle it would be possible to determine current density by establishing the surface area of the part to be plated. Indeed, the procedure is employed either of measuring or determining surface area directly or else with certain parts of known configuration, of establishing surface area from weight. Using the figures for the surface area to be plated which are found, the requisite plating current can be calculated from the desired optimum current density and current can be regulated in the conventional way for substantially the whole of the period of deposition.
In the case of articles of complicated shape whose surface area is difficult to establish, such as are encountered when electro-plating printed-circuit boards or printed circuits for example, it is not practicable to proceed by establishing surface area. It is above all impracticable when fully automatic plating installations are supplied with articles to be plated of a varying nature. Another frequently employed method namely that of obtaining the actual figures for operation from the coating thickness found on a test batch, becomes unusable when, in particular, the number of items to be dealt with is small.
With fully automatic installations it is often the practice to use a constant voltage supply. For this the voltage is regulated to be constant at the feed points to the mounting for the rack for the parts to be plated. However, this method has the drawback that the electrical path between mounting for the rack and the region of cathode surface at which the metal deposition takes place is too long. This means that the deposition potential which is needed at the cathode vis a vis the electrolyte is by no means accurately reflected by what appears at the feed points.
Since the system voltage, i.e. the dropped voltage in the technical system, may be of the same order of magnitude as the deposition potential, then there is certainly nothing to be gained by using constant voltage to regulate current density when, in automatic installations, racks are loaded with different kinds of articles to be plated.
There are also processes known in which the deposition potential at the cathode is measured with the help of an auxiliary electrode by making a high-resistance measurement of potential. This method too is affected by system-dependant factors and is best suited to keeping known deposition conditions constant.
It is an object of the invention to provide a method by which, particularly in automatic electro-plating installations where the kinds of article to be plated vary and their surface area is unknown or largely unknown, the plating current or voltage can be so adjusted that a current density which is indicated as the optimum for the electrolyte in use can be achieved and maintained during the period of deposition. A further object is to provide apparatus suitable for putting the method into effect.